Optical fibers are in widespread use today as transmission media because of their large bandwidth capabilities and small size. Developments in the optical fiber communications filed have been rapid. Although desired for their large bandwidth capabilities and small size, light-transmitting optical fibers are mechanically fragile, exhibiting low-strain fracture under tensile loading and degraded light transmission when bent. The degradation in transmission which results from bending is referred to as macrobending and microbending losses. As a result, cable structures have been developed to protect mechanically the optical fibers in various environments. For example, tortuous or arcuate paths for an optical cable necessitate increased pulling loads which result in higher stresses.
Cable structures which have been developed for optical fibers include loose tube, and loose bundle cables. In one early type of optical communications cable, a plurality of optical fibers not exceeding twelve is enclosed in an extruded plastic tube to form a unit called a loose tube. A plurality of these tubed units is enclosed in a common extruded plastic tube which is enclosed in a sheath system. Each unit is made on a manufacturing line and inventoried until it is stranded with other units on another line whereat a plastic jacket also is applied.
What still was sought was a cable for optical fiber transmission which departed from the stranding of units and which inhibited the introduction of undue stresses that could lead to bending losses in the optical fibers. A cable which satisfies these needs is disclosed in App. Ser. No. 721,533 which was filed on Apr. 10, 1985, in the names of C. H. Gartside, III, A. J. Panuska, and P. D. Patel. That cable includes a plurality of optical fibers which are assembled together in a core without intended stranding to form units which extend in a direction along a longitudinal axis of the cable and which are referred to as a loose bundle. A length of tubing which is made of a plastic material encloses the plurality of units and is parallel to the longitudinal axis of the cable. The ratio of the cross-sectional area of the plurality of optical fibers to the cross-sectional area within the tubing is controlled.
A sheath system for the just-described cable may be one disclosed in U.S. Pat. No. 4,241,979 which issued on Dec. 30, 1980 in the names of P. F. Gagen and M. R. Santana. A bedding layer, about which strength members are wrapped helically, is added between plastic extruded inner and outer jackets to control the extent to which the strength members are encapsulated by the outer jacket. The cable includes two separate layers of metallic strength members, which are wrapped helically in opposite directions. Under a sustained tensile load, these two layers of strength members can be designed to produce equal but oppositely directed torques about the cable to insure the absence of twisting. Advantageously, the strength members not only provide the necessary strength characteristics for the cable, but also reinforce the sheath and help protect the optical fiber from external influences. Such a sheath system may be replaced with one in which only one layer of metallic strength members is used. See U.S. Pat. No. 4,765,712 which issued on Aug. 23, 1988, in the names of W. D. Bohannon, Jr., et al.
Embedding strength members in the sheath layers rather than the cable center also provides a composite reinforced tube which results in a compact constuction and enhances fiber protection. The strength members in the above-described sheaths are helically applied within the jacket for flexibility and stability during bending, allowing a tight bend radius without kinking.
In the prior art, metallic wires of the hereinbefore-mentioned Gagen-Santana cable sheath, which is referred to as a cross-ply sheath, have been replaced with glass fiber, rod-like members. The rod-like members are capable of withstanding expected compressive as well as tensile loading. Compressive loading occurs when the cable contracts during the initial shrinkage of the jacket material and during thermal cycling. However, the replacement of the metallic strength members with glass rods increases the cost of the cable and stranding of the rods continues to require a relatively low manufacturing line speed.
Although the sheath systems of U.S. Pat. Nos. 4,241,979 and 4,765,712 meet many customer needs, efforts have continued to find alternatives in order to provide enhanced sheath entry. The number of strength members in prior art cables is usually high. As optical fiber transmission becomes more widely used in the loop distribution network, frequent sheath entry into a tapered network for purposes of splicing will be required. Cables to be employed in the so-called loop must provide for ease of entry. If a cable includes strength members in its sheath, their number must be minimized if at all possible while continuing to provide suitable strength characteristics.
A cable which provides enhanced, or express entry as it is called, into the core is one described in U.S. patent application Ser. No. 036,954 which was filed on Apr. 10, 1987, in the names of M. D. Kinard, et al. That optical fiber cable includes a core which comprises at least one optical fiber and a tubular member which may be made of a plastic material and which encloses the core. A jacket which is made of a plastic material encloses this tubular member. In a preferred embodiment, the cable also includes a strength member system which includes two diametrically opposed, linearly extending metallic strength members which are disposed adjacent to the tubular member, and which extend parallel to a longitudinal axis of the core. Because the strength members are not wound helically about the tubular member, the manufacturing process need not involve the rotation of relatively heavy supplies. The strength members have sufficient tensile and compressive stiffnesses and are coupled sufficiently to the jacket to provide a composite structure which is effective to inhibit contraction of the cable and to provide the cable with suitable strength properties.
Also, there has been a long felt need for an all-dielectric cable construction. Such a cable which could be run from building ducts to service distribution points would obviate the need for grounding connections at splice points that add to the cost of cable installations. Further, a cable such as the sought-after, all-dielectric cable would decrease substantially the probability of lightning strikes.
Another consideration relates to the size of the transverse cross-sectional area of the cable. If each of the two metallic strength members of a cable disclosed in priorly identified application Ser. No. 036,954 were replaced with one made of a dielectric material, its transverse cross-sectional area would increase to such an extent that the transverse cross-sectional area of the cable would have to be increased.
In U.S. Pat. No. 4,743,085 which issued on May 10, 1988 in the names of A. C. Jenkins and P. D. Patel, a cable includes two layers of dielectric strength members within its sheath system with all the members of the inner layer and some of the outer layer being relatively flexible. The remaining strength members of the outer-layer, which are relatively stiff, are capable of withstanding expected compressive as well as tensile loading. Compressive loading occurs when the cable tends to contract during initial shrinkage of the jacket material, during bending, and during thermal cycling. Although this cable is all-dielectric, it includes many strength members and lacks the express sheath entry feature which is desired for local area network usage.
What is needed and what does not appear to be available in the prior art is an all-dielectric cable which has desired features and which is relatively cost effective to manufacture. Desirably, strength members of the sought-after cable are disposed in its sheath system. Further, inasmuch as such a cable would be a strong candidate for use in the local loop, the sought-after cable should have a sheath system which facilitates express core entry.